Multi-length cyclic prefix for OFDM transmission in PLC channels

ABSTRACT

Embodiments of the invention provide multiple cyclic prefix lengths for either both the data-payload and frame control header or only the data payload. Frame control header (FCH) and data symbols have an associated cyclic prefix. A table is transmitted in the FCH symbols, which includes a cyclic prefix field to identify the cyclic prefix length used in the data payload. A receiver may know the cyclic prefix length used in the FCH symbols in one embodiment. In other embodiments, the receiver does not know the FCH cyclic prefix length and, therefore, attempts to decode the FCH symbols using different possible cyclic prefix lengths until the FCH symbols are successfully decoded.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a Continuation of U.S. patent application Ser. No.15/227,367, filed Aug. 3, 2016, of and claims the benefit to U.S. patentapplication Ser. No. 13/781,452, filed on Feb. 28, 2013, which claimsthe benefit of provisional U.S. Patent Application Ser. No. 61/604,610,filed Feb. 29, 2012, all of which are incorporated herein by thisreference.

BACKGROUND

Embodiments of the invention are directed, in general, to Power LineCommunication (PLC) systems and, more specifically, OFDM transmissionsin PLC channels.

Various OFDM-based standards exist for narrowband power-linecommunications, such as the G3-CENA, G.hnem, IEEE-P1901.2 standards. Ineach of these standards, the OFDM system is designed assuming a fixedcyclic prefix length.

In some PLC communication links, it is envisioned that the channel delayspread may be significantly different from the cyclic prefix lengthspecified for the standard in use. If the channel delay spread is longerthan the cyclic prefix, this will result in an signal-to-noise ratio(SNR) loss in the received data. If the channel delay spread is shorterthan the cyclic prefix, this will result in a reduction in channelthroughput which may be significant for long packet lengthtransmissions.

Power line communications (PLC) include systems for communicating dataover the same medium that is also used to transmit electric power toresidences, buildings, and other premises, such as wires, power lines,or other conductors. In its simplest terms, PLC modulates communicationsignals over existing power lines. This enables devices to be networkedwithout introducing any new wires or cables. This capability isextremely attractive across a diverse range of applications that canleverage greater intelligence and efficiency through networking. PLCapplications include utility meters, home area networks, lighting, andsolar.

PLC may also serve as an important enabling technology for the massdeployment of solar equipment by providing a communication channel tosolar inverters for monitoring and managing power across the grid byutility companies. While radio frequency (RF) communications have madesome progress in solar installations, PLC offers an ideal means forconnecting equipment with high reliability and at a low cost on DC or AClines.

PLC is a generic term for any technology that uses power lines as acommunications channel. Various PLC standardization efforts arecurrently in work around the world. The different standards focus ondifferent performance factors and issues relating to particularapplications and operating environments. Two of the most well-known PLCstandards are G3 and PRIME. G3 has been approved by the InternationalTelecommunication Union (ITU). IEEE is developing the IEEE P1901.2standard that is based on G3. Each PLC standard has its own uniquecharacteristics. PRIME is designed for low voltage lines with low noiseand targets higher data rates. On the other hand, G3 is designed formedium voltage lines and targets lower data rates.

SUMMARY

Systems and methods for implementing multiple cyclic prefix lengths inpower line communications (PLC) are described. Multiple cyclic prefixlengths may be used for both the data payload and frame control headeror for the data payload alone.

A typical OFDM transmission packet comprises a preamble, header, anddata symbols. The header and data symbols are OFDM symbols with anassociated cyclic prefix. A frame control header (FCH) table istransmitted in the header symbols and includes a cyclic prefix fieldspecifies the cyclic prefix length used in the data payload. The numberof bits used in the cyclic prefix field determines the number of cyclicprefix length options available.

When decoding the FCH symbols, two options are possible. In one case,the FCH cyclic prefix length is always fixed and maybe different fromthe data cyclic prefix length. If the FCH cyclic prefix length is fixed,then it is always known to the receiver. In this case, there is noambiguity for the receiver when decoding the FCH. In other cases, theFCH cyclic prefix length is the same as the data symbols cyclic prefixlength but is unknown to the receiver. In this case, the receiver willhave to decode the FCH symbols assuming various cyclic prefix hypothesesand then select the hypothesis that results in a CRC pass at the end ofthe FCH decoding.

In some embodiments, one or more of the methods described herein may beperformed by one or more PLC devices (e.g., a PLC meter, PLC dataconcentrator, etc.). In other embodiments, a tangible electronic storagemedium may have program instructions stored thereon that, upon executionby a processor within one or more PLC devices, cause the one or more PLCdevices to perform one or more operations disclosed herein. Examples ofsuch a processor include, but are not limited to, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), asystem-on-chip (SoC) circuit, a field-programmable gate array (FPGA), amicroprocessor, or a microcontroller. In yet other embodiments, a PLCdevice may include at least one processor and a memory coupled to the atleast one processor, the memory configured to store program instructionsexecutable by the at least one processor to cause the PLC device toperform one or more operations disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention(s) in general terms, reference willnow be made to the accompanying drawings, wherein:

FIG. 1 illustrates a typical OFDM transmission packet.

FIG. 2 is a high level block diagram of an OFDM transmitter forgenerating OFDM packets according to one embodiment.

FIG. 3 illustrates the addition of the cyclic prefix according to oneembodiment.

FIG. 4 illustrates an OFDM transmission packet incorporating aspects ofthe present invention.

FIG. 5 is a flowchart illustrating a method or process for generatingsymbols in an OFDM modulator according to one embodiment.

FIG. 6 is a flowchart illustrating a method or process for decodingsymbols received by a receiver according to one embodiment.

DETAILED DESCRIPTION

The invention now will be described more fully hereinafter withreference to the accompanying drawings. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art. Oneskilled in the art may be able to use the various embodiments of theinvention.

FIG. 1 illustrates a typical OFDM transmission packet 100 comprising apreamble 101, frame control header (FCH) symbols 102, and data symbols103. The preamble 101 typically consists of repeated sequences, such asa series of SYNCP and SYNCM symbols, which are used for automatic gaincontrol adaptation, symbol synchronization, channel estimation, andinitial phase reference estimation. FCH symbols 102 and data symbols 103are OFDM symbols with an associated cyclic prefix.

FIG. 2 is a high level block diagram of an OFDM transmitter 200 forgenerating OFDM packets 100 according to one embodiment, such as forcircuits complying with the IEEE P1901.2 standards. FCH bits 201 anddata bits 202 are scrambled, encoded, and interleaved in a forward errorcorrection (FEC) encoder 203. An OFDM signal is generated in a modulator204 by performing inverse fast Fourier transform (IFFT) on the encodedbits. OFDM modulator 204 adds a cyclic prefix (CP) 205 to the output ofthe IFFT block 206. The output of the OFDM modulator 204 is provided topower line circuit 207, such as a medium voltage (MV) or low voltage(LV) power line, thorough analog front end (AFE) 208 and couplingcircuitry 209. The coupling circuitry may include, for example, linedrivers, transformers, filters, and MV/LV couplers.

In known power line communication systems, OFDM modulators add a cyclicprefix of predetermined size to both the FCH symbols and data symbols.In one embodiment, such as systems complying with the IEEE P1901.2standards, the last 30 samples at the output of the IFFT block 206 areplaced at the front of the symbols by add CP block 205.

FIG. 3 illustrates the addition of the cyclic prefix according to oneembodiment. The IFFT block 206 generates 256 time-domain OFDM symbolsamples (301). The add CP block (205) takes a group of samples (302)from the end of the 256 OFDM symbol samples (301) and copies them to thefront of the 256 OFDM samples as a cyclic prefix 303 to create OFDMsymbol 304 for transmission. In one embodiment, the last 30 samples(302) are used as the cyclic prefix. However, in other embodiments, anynumber of samples (302) may be used as the cyclic prefix.

In a system operating at 400 k samples per second, such ascommunications in the CENEILC A band, an OFDM symbol with 256 samples is640 μs long (i.e., 256 samples/400 k samples-per-second). When a30-sample cyclic prefix is added, then the OFDM symbol becomes 286samples long, which corresponds to 715 μs long (i.e., 286 samples/400 ksamples-per-second). The extra 30 samples make the OFDM symbol 75 μslonger (i.e., 30 samples/400 k samples-per-second).

When the delay spread increases, it would be desirable to adjust thesize of the cyclic prefix to compensate for echoes and delays in thecommunication channel. Embodiments of the invention allow the system toselect different cyclic prefix lengths for the FCH and the data symbols.The same cyclic prefix length may be used for both the FCH and the datasymbols, or different cyclic prefix lengths may be used for each the FCHsymbols and the data symbols.

When the transmitter can select between different cyclic prefix lengths,the receiver has to determine what cyclic prefix length was used inorder to properly decode the received OFDM symbols. Additionally, thereceiver must determine if the same or different cyclic prefix lengthswere used for the FCH symbols and the data symbols.

Data Payload Decoding

The frame control header (FCH) table, which is transmitted in the FCHsymbols, has information of the various PHY layer parameters in the datapayload, such as modulation information, frame length/number of datasymbols, tone map information, etc. In one embodiment, a cyclic prefixfield is also included in the FCH table. The cyclic prefix field mayspecify the cyclic prefix length that is used in the data payload. Thecyclic prefix length may be defined in terms of the cyclic prefixduration or number of samples comprising the cyclic prefix.

Table 1 illustrates an example cyclic prefix field using a two-bit fieldin the FCH table. The two-bit field supports four different cyclicprefix lengths as shown in Table 1. Each value of the cyclic prefixfield corresponds to a different cyclic prefix length that may be usedin the corresponding data symbols.

TABLE 1 FCH Table Cyclic Prefix Field Cyclic Prefix 00 X μs 01 Y μs 10 Zμs 11 W μs

Table 2 illustrates a one-bit cyclic prefix field in the FCH table. Witha one-bit cyclic prefix field, only two cyclic prefix lengths may besupported.

TABLE 2 FCH Table Cyclic Prefix Field Cyclic Prefix 0 X μs 1 Y μs

In the example shown in Tables 1 and 2, the length of the cyclic prefixis depicted in micro-seconds; however, it will be understood that thelength may also be represented as a number of samples in otherembodiments.

FCH Decoding

At least the following options are possible at the receiver whendecoding the FCH symbols if the FCH cyclic prefix length is unknown.

In one embodiment, the FCH cyclic prefix length is always fixed;however, the FCH cyclic prefix length may be different from the datacyclic prefix length. Since the FCH cyclic prefix length is fixed, it isalways known to the receiver. In this case, there is no ambiguity forthe receiver when decoding the FCH symbols. Once the FCH symbols aredecoded, the data cyclic prefix length may be read from the FCH tableand that value used to decode the data symbols.

In another embodiment, the FCH cyclic prefix length is the same as thedata symbols' cyclic prefix length. In this case, the receiver decodesthe FCH symbols assuming various cyclic prefix length possibilities andchooses the possibility that results in a successful CRC pass at the endof the FCH decoding. For example, referring to the example of Table 1above, if there are four possible cyclic prefix lengths, then thereceiver may attempt to decode the FCH using each of the possible cyclicprefix lengths in turn until the FCH is successfully decoded.

FIG. 4 illustrates an OFDM transmission packet 400 incorporating aspectsof the present invention. Packet 400 comprises preamble 401, framecontrol header (FCH) segment 402, and data segment 403. Frame controlheader segment 402 comprises a plurality of FCH symbols 404 separated byguard intervals (GI) 405. Each of the FCH symbols 404 comprises a FCHsegment 406 and an FCH cyclic prefix (CP) segment 407. Data segment 403comprises a plurality of data symbols 408 separated by guard intervals(GI) 409. Each of the data symbols 408 comprises a data segment 410 anda data cyclic prefix (CP) segment 411.

The lengths of the FCH CP 407 and data CP 411 may be the same in oneembodiment or may be different in other embodiments. The size of thedata CP 411 may be encoded in an FCH table carried by FCH symbols 402.In some embodiments, the length of FCH CP 407 is predetermined and knownto the receiver. The receiver uses the known FCH CP length when decodingthe FCH symbols 404. When the FCH symbols 402 are all decoded, the valueof the data CP 411 length is read from the FCH table and used to decodedata symbols 403.

In other embodiments, the lengths of the FCH CP 407 and data CP 411 arethe same, but unknown to the receiver. The receiver may attempt todecode FCH symbols 404 using known options for the CP length until theFCH symbols are successfully decoded. The FCH CP length value thatresults in successful FCH symbol decoding is then used to decode thedata symbols 403.

Cyclic Prefix Selection in PLC

The transmitter may select the cyclic-prefix length based oncharacteristics of the PCL network. The cyclic prefix may be selected,for example, so that multi-path in the PCL network is within the cyclicprefix.

Deployment-Dependent Cyclic Prefix. The length of the cyclic prefix maybe selected based on PLC network deployment characteristics. Multi-patharises in the PLC network due to signal reflections at terminationpoints in the power lines. Thus, the type of cabling that is used, thelength of the cables, the number of branch points, and similar factorscan determine the amount of multi-path that is to be expected in the PLCnetwork. In some deployments, the utility provider has accurate maps ofthe power-line communication links as well as information on the cablingused in the network. This information may be used to compute theexpected amount of multi-path on the channel for different links. Inthis scenario, the cyclic-prefix length may be a network-configuredparameter based on the expected amount of multi-path that is computedbased on the PLC network topology and materials. Using this knowledge ofthe network topology and materials, the amount of multi-path expectedfor a given transmitter can be estimated and then used to determine anappropriate cyclic prefix for that transmitter.

Load-Dependent Cyclic Prefix. The amount of electrical load in the powerline network may also determine the amount of multi-path experienced onthe communication channel. When the electrical load is high in thenetwork, reflections and multi-path may increase in the communicationchannel. Therefore, when there is high electrical loading on the PLCnetwork, a longer cyclic prefix may be required for acceptablecommunication.

Time-Dependent Cyclic Prefix. High electrical loads typically occur in aresidential environment during mid-day and evening hours, such as whenmany lights and appliances are in use. Additionally, electrical usevaries depending upon the season, such as increased electrical loads inthe summer due to air conditioning use. As a result, the cyclic prefixlength used by a PLC transmitter may be time-dependent to account forvarying electrical loads in the network. A utility provider may havehistorical use information that would allow it to estimate theelectrical load at different times, days, or months. Using this expectedload data, cyclic prefixes may be pre-calculated for the transmitter.The transmitter may then select the cyclic prefix to use depending uponthe time, day, and/or month.

FIG. 5 is a flowchart illustrating a method or process for generatingsymbols in an OFDM modulator for a power line communication (PLC) deviceaccording to one embodiment. In step 501, FCH symbols and data symbolsare generated from encoded FCH and data bits. In step 502, a data cyclicprefix is prepended to the data symbols. The length of the data cyclicprefix may be selected based upon a channel delay characteristic. Instep 503, an FCH cyclic prefix is prepended to the FCH symbols. Thelength of the FCH cyclic prefix may be the same as the length of thedata cyclic prefix. Alternatively, length of the FCH cyclic prefix maybe a predetermined value that is different from the length of the datacyclic prefix.

In step 504, the length of the data cyclic prefix is identified in anFCH table carried in the FCH symbols. The length of the data cyclicprefix may be identified, for example, by setting one or more bits in acyclic prefix field of the FCH table.

FIG. 6 is a flowchart illustrating a method or process for decodingsymbols received by a receiver in a power line communication (PLC)device according to one embodiment. In step 601, packets comprising FCHsymbols and data symbols are received. In step 602, the FCH symbols aredecoded. The FCH symbols may be decoded using a predetermined FCH cyclicprefix length in one embodiment. In other embodiments, the FCH symbolsare decoded using one or more known FCH cyclic prefix length valuesuntil the FCH symbols are successfully decoded.

In step 603, an FCH table is extracted from the decoded FCH symbols. Instep 604, the length of a data cyclic prefix is determined from the FCHtable. For example, the length of the data cyclic prefix may bedetermined from one or more bits in a cyclic prefix field of the FCHtable. The data symbols are decoded in step 605 using the data cyclicprefix length. In one embodiment, the length of the FCH cyclic prefix isdifferent than the length of the data cyclic prefix. In otherembodiments, the lengths of the FCH cyclic prefix and the data cyclicprefix are the same, but not initially known to the receiver.

Many modifications and other embodiments of the invention(s) will cometo mind to one skilled in the art to which the invention(s) pertainhaving the benefit of the teachings presented in the foregoingdescriptions, and the associated drawings. Therefore, it is to beunderstood that the invention(s) are not to be limited to the specificembodiments disclosed. Although specific terms are employed herein, theyare used in a generic and descriptive sense only and not for purposes oflimitation.

We claim:
 1. A method, comprising: performing, by a receiver in a powerline communication (PLC) device: receiving packets that include framecontrol header (FCH) symbols and data symbols; decoding the FCH symbols:determining a length of a data cyclic prefix for the data symbols from aFCH table; and decoding the data symbols using the data cyclic prefixlength from the FCH table.
 2. The method of claim 1, further comprising:decoding the FCH symbols using a predetermined FCH cyclic prefix length.3. The method of claim 1, wherein a length of a FCH cyclic prefix is thesame as the data cyclic prefix length.
 4. A powerline communications(PLC) device, comprising: a receiver configured to receive and decodepackets that include frame control header (FCH) symbols and datasymbols, the receiver further configured to determine a length of a datacyclic prefix from the decoded FCH symbols, the receiver furtherconfigured to decode the data symbols using the data cyclic prefixlength.
 5. The PLC device of claim 4, wherein the receiver decodes theFCH symbols using a predetermined FCH cyclic prefix length.
 6. The PLCdevice of claim 4, wherein a length of a FCH cyclic prefix is differentthan the length of the data cyclic prefix.
 7. The PLC device of claim 4,wherein a length of a FCH cyclic prefix is the same as the data cyclicprefix length.
 8. The PLC device of claim 4, wherein the receiverdetermines the length of the data cyclic prefix from a one-bit prefixfield.
 9. A power line communication (PLC) device for sending andreceiving data over a PLC network, comprising: a processor; atransmitter coupled to the processor; a receiver coupled to theprocessor; and a memory coupled to the processor, the memory storingprogram instructions that, when executed by the processor, cause the PLCdevice to: generate frame control header (FCH) symbols and data symbolsfrom encoded FCH and data bits; prepend a data cyclic prefix to the datasymbols, a length of the data cyclic prefix is selected based upon acharacteristic of the PLC network; and identify the length of the datacyclic prefix in an FCH table carried in the FCH symbols.
 10. The PLCdevice of claim 9, wherein the program instructions, when executed bythe processor, cause the PLC device to also prepend a FCH cyclic prefixto the FCH symbols.
 11. The PLC device of claim 10, wherein a length ofthe FCH cyclic prefix is a predetermined value that is different thanthe length of the data cyclic prefix.
 12. The PLC device of claim 9,wherein identifying the length of the data cyclic prefix in the FCHtable carried in the FCH symbols includes setting a one-bit cyclicprefix field in the FCH table to identify the length of the data cyclicprefix.
 13. The PLC device of claim 9, wherein the characteristic of thePLC network that is used to select the length of the data cyclic prefixis a PLC network deployment characteristic.
 14. The PLC device of claim9, wherein the characteristic of the PLC network that is used to selectthe length of the data cyclic prefix is an electrical load in a PLCnetwork.
 15. The PLC device of claim 9, wherein the characteristic ofthe PLC network that is used to select the length of the data cyclicprefix is time dependent.
 16. A power line communication (PLC) devicefor sending and receiving data over a PLC network, comprising: aprocessor; a transmitter coupled to the processor; a receiver coupled tothe processor; and a memory coupled to the processor, the memory storingprogram instructions that, when executed by the processor, cause the PLCdevice to: receive packets that include frame control header (FCH)symbols and data symbols; decode the FCH symbols; determine a length ofa data cyclic prefix from the FCH symbols; and decode the data symbolsusing the data cyclic prefix length.
 17. The PLC device of claim 16,wherein decoding the FCH symbols includes decoding the FCH symbols usinga predetermined FCH cyclic prefix length.
 18. The PLC device of claim16, wherein the length of the data cyclic prefix is different than alength of a FCH cyclic prefix.
 19. The PLC device of claim 16, whereinthe length of the data cyclic prefix is the same as a FCH cyclic prefixlength.
 20. The PLC device of claim 16, wherein the receiver determinesthe length of the data cyclic prefix from a one-bit prefix field.